Carbide Transformation Research Articles

Effect of carbides on high-temperature aging embrittlement in 12%Cr martensitic heat-resistant steel

It is well-known that the 9–12%Cr martensitic heat-resistant steels own excellent mechanical properties; yet, in the case of aging embrittlement mechanism, little has been documented with carbides effect. In this study, to understand the aging embrittlement mechanism in steel 10Cr12Ni3Mo2VN, we focused on high-temperature short-term aging embrittlement by investigating the relationship between the heat treatment, microstructures, and aging embrittlement. It is mainly interested in the effect of carbides on the impact toughness of the steel sample after a heat treatment. The results show that the steel appears a reversible short-term aging embrittlement at temperatures within 500–600℃. It was found that carbides play an important role in the aging embrittlement, especially the M7C3-type carbides. Carbides formed at boundaries cause boundaries weakening and induce tempered martensite matrix to fracture in quasi-cleavage mode, which leads to the aging embrittlement. It was also found that the aging embrittlement can be eliminated by re-tempering at a higher temperature due to the transformation of carbides from M7C3-type to M23C6-type. Aging embrittlement arises again but becomes weak after a re-aging treatment. In addition, by refining prior austenite grains, increasing temper temperature and reducing carbon content, it can improve the resistance to aging embrittlement in the sample as refining grains increase the resistance to brittle fracture; what’s more, it could reduce the precipitation of carbides at boundaries during aging by increasing temper temperature and reducing carbon content.

Aging Phenomena during In-Service Creep Exposure of Heat-Resistant Steels

An investigation of aging phenomena during creep exposure has been conducted for HP-Nb cast reformer tubes for several exposure conditions. Aging was manifested by carbide precipitation, carbide coarsening, and carbide transformation. The transformation of primary M7C3 to the more stable M23C6 carbide takes place at high exposure temperature (910 °C and above). The primary MC carbides transform to the Ni-Nb silicide or G-phase during creep exposure. The presence of Ti in the steel prevented the transformation of MC carbides to the G-phase. Morphological changes like needle to globular transitions, rounding of carbide edges, and carbide coarsening take place during creep exposure. The room-temperature tensile elongation and ultimate tensile strength are significantly reduced during creep exposure. The above aging phenomena are precursors to creep damage.

Research on the Main Controlling Factor of Transformation Among M3C, M7C3 and M23C6 Carbides

Based on the limitations of traditional viewpoint that the carbide types in chromium wear-resistant alloys were determined by the Cr/C value, the aim of this study was to reveal the mechanism of recombination-reconstruction between different atoms and the influence of various elements on the formation of carbide types during the mutual transformation of M3C, M7C3 and M23C6 carbides by analyzing the difference of carbide types in different Fe-Cr-C alloys and the transformation characteristics of carbide types before and after carburization and decarbonization of 3Cr13 stainless steel. The viewpoints were put forward as follows: the transformation between M3C and M7C3 carbides are predominantly controlled by the chromium content, and the main controlling factor of transformation between M7C3 and M23C6 carbides is the carbon content. In the different stages of carbide transformation, the chromium and carbon play the main controlling role respectively

G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube

HP50M heat-resistant cast austenitic steel tubes are exposed to high temperatures (i.e. 900 °C) for long periods (10 years or longer) in petrochemical reforming plants. Twenty-years aged tubes exhibit microstructural changes such as primary carbide transformation and growth, MC carbide transition to G-phase, and fine secondary M23C6 precipitation. Analytical electron microscopy reveals that silicon tends to segregate at austenite inter-dendritic boundaries in fresh as-cast tubes. Thermo-kinetic simulation of long-term precipitate evolution of the steel is carried out together with Si segregation compositions. The experimentally observed general trend that G-phase forms at the expense of MC carbide is successfully predicted for the Si segregation compositions after a few thousand hours of aging in a temperature range between 700 and 900 °C. This study demonstrates that the Si segregation plays a crucial role in the formation of G-phase in this type of heat-resistant cast steels. Additional heat-treatment experiment confirms that G-phase becomes unstable above 900 °C and the reverse transition to MC occurs.

Gleeble-Simulated and Semi-Industrial Studies on the Microstructure Evolution of Fe-Co-Cr-Mo-W-V-C Alloy during Hot Deformation.

Fe-Co-Cr-Mo-W-V-C alloy is one of the most important materials for manufacturing drills, dies, and other cutting tools owing to its excellent hardness. However, it is prone to cracking due to its poor hot ductility during continuous hot working processes. In this investigation, the microstructure characteristics and carbide transformations of the alloy in as-cast and wrought states are studied, respectively. Microstructural observation and first-principles calculation were conducted on the research of types and mechanical properties of carbides. The results reveal that carbides in as-cast Fe-Co-Cr-Mo-W-V-C alloy are mainly Mo2C, VC, and Cr-rich carbides (Cr7C3 and Cr23C6). The carbides in wrought Fe-Co-Cr-Mo-W-V-C alloy consist of Fe2Mo4C, VC, Cr7C3, and a small amount of retained Mo2C. For these carbides, Cr7C3 presents the maximum bulk modulus and B/G values of 316.6 GPa and 2.48, indicating Cr7C3 has the strongest ability to resist the external force and crack initiation. VC presents the maximum shear modulus and Yong’s modulus values of 187.3 GPa and 465.3 GPa, which means VC can be considered as a potential hard material. Hot isothermal compression tests were performed using a Gleeble-3500 device to simulate the flow behavior of the alloy during hot deformation. As-cast specimens were uniaxially compressed to a 70% height reduction over the temperature range of 1323–1423 K and strain rates of 0.05–1 s−1. A constitutive equation was established to characterize the relationship of peak true stress, strain rate, and deformation temperature of the alloy. The calculated results were in a good agreement with the experimental data. In order to study the texture evolution, the microstructures of the deformed specimens were observed, and an optimal deformation temperature was selected. Using the laboratorial optimal temperature (1373 K) in forging of an industrial billet resulted in uniform grains, with the largest size of 17 µm, surrounded by homogenous spherical carbides.

Primary carbide transformation in a high performance micro-alloy at 1000 °C

The transformation from M7C3 → M23C6 in a high performance micro alloy (HP-MA) was studied using different techniques. Following the cooling during the centrifugal casting, the as-cast HP-MA alloy consists of an austenitic matrix with the primary carbide network consisting of a combination of M7C3 (M being mainly chromium) and MC (M being mainly niobium). During the heat treatment at 1000 °C, the primary chromium carbides transform from M7C3 to M23C6. The incompletely transformed primary carbide consists of an outer shell of M23C6 carbide (coherent with the austenitic matrix) and a core of M7C3 type carbide. The experimentally determined M23C6 shell thickness agrees reasonably well with the estimated diffusion distance of carbon in M23C6 based on Zener's relation implying that the M7C3 → M23C6 transformation is mainly controlled by the diffusion of the carbon from M7C3 to the matrix through the M23C6 shell. This mechanism is discussed vis-à-vis the literature reported mechanism on the transformation.

A study of deformation behavior and phase transformation in 4H-SiC during nanoindentation process via molecular dynamics simulation

The deformation behavior and phase transformation of 4H silicon carbide (4H-SiC) during nanoindentation process is investigated with a cube corner diamond indenter through molecular dynamics simulation. It is found through the research that the basal dislocations are most likely to be generated in (0001) face and the indentation process contributes to the distortion of 4H-SiC lattice. In addition, phase transformation from 4H-SiC to 3C-SiC is firstly observed via MD simulations during indentation process. Cross-sectional observation in (12¯10) plain shows that 3C-SiC layers appear firstly during nanoindentation process, and the layers are observed at small indentation depth. 3C-SiC grain is generated based on 3C-SiC layers, and the transformation is more likely to appear at larger indentation depth. The phase transformation from 4H-SiC to 3C-SiC results from the shear stress induced by indenter during loading process. 3C-SiC grain and layers are both generated from the slip of 3C seeds under the influence of shear stress, and the condition of 3C-SiC grain formation is stricter. Moreover, the P-h curve is studied and the vertical deformation mode during indentation process on 4H-SiC can be reflected on P-h curve as small pop-in events. The findings are meaningful for the study of deformation mechanism of SiC and the application of SiC in precision machining.

Phase evolution and densification behavior of MIM418 superalloy utilizing master alloy approach

MIM418 superalloy was prepared by metal injection molding of master alloy powder and carbonyl Ni powder. Phase formation process and its evolution during heating process was followed by high temperature x-ray diffraction, and the relationship between phase transition, diffusion process and the densification behavior was discussed in detail. Microstructural characteristics of the samples during sintering process and the densification mechanism were investigated. Asynchronous diffusion of dissolvent elements, γʹ phase and carbides were observed. Severe outward diffusion of γʹ forming elements (Al) from the master alloy to the nickel powder leads to the precipitation of a large population of γʹ phase in the temperature range of 700–900 °C. The mass transfer of γ-partitioning elements (Cr, Mo) almost accomplished at 1100 °C. The phase transformation of carbides evolves from non-equilibrium MCʹ carbides in the master alloy powder to M23C6 carbides at 900–1100 °C, and finally to thermodynamically stable MC carbides in the homogenized sample. Accelerated diffusion and densification process were observed during transient liquid phase sintering process. The relatively density achieves 98.4% and ultimate strength (1047 MPa) is comparable with that of the prealloyed sample.

Microstructures and high-temperature mechanical properties of a directionally solidified Ni-based superalloy: Influence of boron content

The effects of boron content on the microstructure and mechanical properties of a directionally solidified Ni-based superalloy were investigated. The increase of boron content significantly increases the amount of γ/γ′ eutectic, impels the formation of multi-phase eutectic-like constituent, and promotes the morphological transformation of MC carbides from rod-like to blocky. After standard heat treatment, the incipient melting occurs in the high boron alloy, which can lower the dissolving temperature of γ/γ′ eutectic. With increasing boron content, the volume fraction of secondary γ′ phase decreases significantly and the homogeneity and alignment of secondary γ′ show a decrease, and the tensile strengths at 900 °C decrease slightly mainly due to the strengthening effect of much more tertiary γ′ phases. The wide γ matrix channel facilitates the movement of dislocations, which is somewhat beneficial to the elongation, while the incipiently melted region (IMR), the decreased volume fraction of secondary γ′ phase and the less homogeneity of secondary γ′ phase are rather harmful to the elongation. Consequently, the elongation is drastically reduced. The existence of a large amount of IMRs is principally responsible for the reduction of the stress rupture property. Tertiary γ′ phases re-dissolving in γ matrix channel facilitates the movement of dislocations between large secondary γ′ phases, which is beneficial to the elongation of the alloys. In addition, the IMRs provide masses of crack sources, and consume the precipitation strengthening elements (Ti, Ta) and the solid solution strengthening elements (Cr, W and Mo), which plays the major role in the degradation of the high-temperature tensile or stress rupture properties.

One Second Formation of Large Area Graphene on a Conical Tip Surface via Direct Transformation of Surface Carbide.

Graphene functionalized nanotips are expected to possess promising potential for various applications based on the outstanding electrical and mechanical properties of graphene. However, current methods, usually requiring a high growth temperature and identical crystal surface to match graphene lattice, are suitable for graphene formation on a flat surface. It remains a big challenge to grow graphene on a nanosized convex surface and fabricate functionalized nanotips with high quality graphene at the apex. In this work, a novel ultrafast annealing method is developed for growing large area graphene on Ni nanotips within 1-2 s. Few layered or multiple layered graphene is presented on the apex or sidewall of the conical tip surface. Direct experimental evidences support that thus-produced graphene is formed via the direct conversion of nickel carbide at the outer surface under the instantaneous high temperature, which is different from the conventional segregation mechanism. This newly developed ultrafast method provides a new route to produce graphene efficiently and economically, promising for both convex surfaces and flat substrates. Moreover, the graphene functionalized nanotips exhibit a great potential for nanoelectrical measurements and conductive scanning probe microscopy (SPM) applications.

Effects of soaking and tempering temperature on microstructure and mechanical properties of 65Si2MnWE spring steel

The heat treatment of soaking, oil quenching and tempering was employed on 65Si2MnWE spring steel. Various soaking and tempering temperatures were attempted to investigate their effects on the microstructure and mechanical properties of 65Si2MnWE steel. The microstructure such as martensite and precipitations were carefully examined, and the tensile, impact and hardness tests were carried out to evaluate the mechanical properties. The results show that the coarsening of martensitic occurs at high soaking temperature due to the facts of precipitation dissolution, dislocation reduction, and also the reduction of reserved austenite. Large amount of alloyed carbide precipitations distribute along grain boundaries after tempering, and these precipitations tried to dissolve into matrix at high tempering temperature. Moreover, with the increase of tempering temperature, the transformation and growth of carbide, and the recovery of martensite simultaneously take place. According to the analysis on mechanical properties, the optimal temperature range for soaking is determined as 850–900 °C, and the optimal tempering temperature should be in the range of 350–400 °C.

Dissolution of M23C6 and New Phase Re-Precipitation in Fe Ion-Irradiated RAFM Steel

The M23C6 precipitate plays a major role in preventing the sliding of the grain boundary and strengthens the matrix in the reduced-activation ferritic/martensic (RAFM) steel. However, its stability might be reduced under irradiation. The microstructural instability of the M23C6 precipitates in the RAFM steels irradiated at 300 °C with Fe ions up to a peak dose of 40 dpa was investigated by transmission electron microscopy. A “Core/Shell” morphology was found for the pre-existing M23C6 and a large number of new small phases appeared in parallel near the periphery of the precipitates after irradiation. The loss of crystallinity of the M23C6 periphery due to the dissolution of carbon atoms into the interface (C-rich “Shell”) actually decreased the size of the Cr-rich “Core”. The new phase that formed around the pre-existing precipitates was M6C (Fe3W3C), which was formed through the carbide transformation of M23C6 to M6C.

Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr–1Mo boiler tube samples were heat treated in the range of 700–950 $$^{\circ }\hbox {C}$$ . Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to $$\hbox {M}_{23}\hbox {C}_{6}$$ type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.

Evaluation of service-induced microstructural damage for directionally solidified turbine blade of aircraft engine

Turbine blades of gas turbine engines usually suffer from severe operational conditions characterized by high temperature and stress. Severe operational conditions during service cause microstructural changes in turbine blades and degrade their mechanical properties. In this study, service-induced microstructural damages in serviced turbine blades manufactured from a directionally solidified superalloy were evaluated. The observed microstructural damage of the turbine blade mainly involves the coarsening and rafting of γ′ precipitates. The leading edge of 60% height of the turbine blades undergone most severe microstructural damage with significant microstructural evolution at this area. Microstructural damage affects the mechanical properties such as Vickers hardness, that is, Vickers hardness decreases as the equivalent diameter decreases. Microstructural damage shows great position-dependent feature as service temperature and radial stress on blade changes. With the aid of energy-dispersive spectrometer (EDS) analysis on carbide, the transformation of carbide does not exist. In addition, no topological closed-packed phase exists in the turbine blade.

Simulation of structure formation in the Fe–C–Cr–Ni–Si surfacing materials

The paper investigates the formation of equilibrium phase state in the surfacing materials 300Cr25Ni3Si3 and 500Cr40Ni40Si2BZr, obtained by electric arc surfacing that employs powder tapes PL AN-101 and PL AN-111. This makes it possible to determine the relationship between chemical composition of the tape and the structure of the formed coating. To examine the patterns of phase transformations during crystallization, we applied thermodynamic simulation using the Thermo-Calc Software. Phase diagrams of the system Fe-Cr-Ni-Si-C were constructed and analyzed; it was established that the alloys in equilibrium belong to the hypereutectic alloys. Their crystallization starts with the formation of primary carbides М 7 С 3 with a hexagonal lattice and ends with the eutectic transformation Liquid ® Austenite + М 7 С 3 The alloy 500Cr40Ni40Si2BZr at a temperature below 1,081 °C undergoes carbide transformations М 7 С 3 «М 3 С 2 . In the alloy 300Cr25Ni3Si3, austenite partially transforms into ferrite in a temperature range below 830 °C. At 600 °C, the alloys consist of the following phases: the alloy 300Cr25Ni3Si3 ‒ austenite, ferrite, and carbide М 7 С 3 ; the alloy 500Cr40Ni40Si2BZr ‒ austenite and carbides М 7 С 3 and M 3 C 2 . It was established that the molar fraction of carbides in the alloy 500Cr40Ni40Si2BZr is considerably higher than that in the alloy 300Cr25Ni3Si3 (56 mol. % and 41 mol. %, respectively). We analyzed a change in the volumetric fraction and phase chemical composition of the alloys depending on temperature. The data obtained make it possible to predict the behavior of alloys under specific operating conditions.

Aplicación del ajuste de Rietveld para correlacionar la evolución microestructural de fundiciones blancas con 18 y 25% en Cromo, templadas en aceite y sucesivos revenidos, con el comportamiento frente al desgaste abrasivo y esfuerzos de flexión

Mediante la aplicación del método de afinamiento estructural de Rietveld se identificaron y cuantificaron las fases presentes en fundiciones blancas hipoeutécticas con 18 y 25% en Cromo, templadas en aceite y tras sucesivos revenidos a 500 ºC. Se correlacionaron los resultados con su comportamiento frente al desgaste abrasivo y bajo esfuerzos de flexión. En ambos casos el porcentaje de austenita retenida tras el temple resultó mínima. Durante los sucesivos revenidos, se constató en la calidad con 18% en Cromo una redisolución parcial de los carburos M7C3 junto a una transformación entre carburos mixtos secundarios M7C3 y M2C. La Martensita tras temple resultó un factor clave frente a la resistencia al desgaste abrasivo, resultando los revenidos tras el temple en aceite desfavorables para esta propiedad. La calidad con 25% en Cromo fue la que presentó un mayor contenido en martensita tras el temple, y por tanto la que mejor comportamiento presentó frente a este tipo de desgaste. Esta misma calidad, tras el doble revenido, fue la que mayor resistencia y capacidad de deformación alcanzó bajo esfuerzos a flexión. La fractura resultó frágil y mayoritariamente transgranular. Sin embargo, pudo observarse la presencia de zonas con fractura dúctil asociadas a martensita doblemente revenida.

Influence of the Nitrogen Content on the Carbide Transformation of AISI M42 High-Speed Steels during Annealing

Attempts were made to elucidate the effect of nitrogen on primary eutectic carbides in as-cast and annealed AISI M42 high-speed steel. Particular emphasis was placed on the transformation of carbides during forging and annealing in steels with different nitrogen concentrations and the influence of final carbides on the impact toughness of the steel. Microstructural observation, electrolytic extraction method, X-ray diffraction analysis, automated inclusion analysis (INCASteel), and impact toughness measurement combined with fractographic observation were conducted on the specimens. Primary M2C carbides were found to be dominant precipitates in the as-cast ingot, with a certain amount of V(C,N). Nitrogen addition promoted the formation of fibrous M2C, whereas lamellar M2C predominated in M42 steel with a low nitrogen concentration (w[N]% = 0.006). Fibrous carbides M2C tend to decompose into more stable carbides M6C and MC during forging and annealing compared to lamellar M2C. Nitrogen alloying only affected the morphologies and dimensions of carbides, but did not change the types of carbides. These improvements in the dimensions and fractions of carbides naturally increased the impact toughness of annealed steel. Hence, it was suggested that the addition of nitrogen to AISI M42 high-speed steel was required to achieve homogeneous distribution of carbides and sufficient impact toughness.

Analysis of phase transformations in Inconel 738C alloy after regenerative heat treatment

Study is based on the characterization of the chemical composition the phase transformations in Inconel 738C gas turbine blade after standard regenerative heat treatment. The microstructure and chemical composition were examined by scanning electron microscope and transmission electron microscope equipped with an energy dispersive X-ray spectrometer. It was found the degradation of microstructure of the blade feather. Redistribution of the chemical elements decreasing the corrosion resistance was observed inside the blade feather. The carbide transformation and sigma phase were found in the structure of the blade feather. It is found that the standard regenerative heat treatment of the IN738 operative gas turbine blade does not effect on carbides transformation, TCP σ-phase dissolution, and thus do not guarantee the full recovery of the IN738 gas turbine blade.

MICROSTRUCTURAL CRITERIA FOR DETERMINING THERMO-MECHANICAL CONDITIONS OF OPERATIONAL VULNERABILITY OF THE WORKING BLADES OF HIGH-TEMPERATURE GAS TURBINE AVIATION ENGINES

The necessity of studying the structure of the blade material and corresponding in-service structural changes is attributed, on the one hand, to the fact that the structural changes may impair the performance of the material thus promoting the blade fracture. On the other hand, the structural changes can serve as diagnostic signs in determination of thermo-mechanical conditions of the blade destruction. Moreover, the structural changes in the blade material often determine the service life of the blades: aside of creep mechanism, in-service depletion of the blade lifetime in high temperature gas turbine engines of maneuvering aircrafts is attributed to complex thermo-mechanical loading of the blades which, in turn activates the mechanisms of fatigue and thermo-mechanical fatigue. Monitoring of the structural state of the blade material with equiaxed crystallization (ZhS6U, VZhL12U), directed and monocrystallization (ZhS26, ZhS32) and studying the features of structural changes attributed to the operational vulnerability of the turbine blade material are the main goals of the study. The features of changes in the morphology of the strengthening γ’-phase and carbide transformation in the blade material that occur in-service are considered. The results of monitoring the structural state of the blade material are compared with the results of testing the same samples under laboratory conditions simulating thermo-mechanical loading. Microstructural criteria are recommended for more accurate evaluation of thermo-mechanical conditions of operating vulnerability of the turbine blades of high-temperature aircraft engines.

New Insights into the Kinetics of Structural Transformation and Hydrogenation Activity of Nano-crystalline Molybdenum Carbide

Transformation of cubic molybdenum carbide into hexagonal β-Mo2C was found to follow the Avrami–Erofeev kinetic model with an n parameter of four indicating a slow nucleation step with a polyhedral growth of the transformed phase. The transformation not only occurred at 773 K under atmospheric H2 pressure, but also imperceptibly occurred under the employed hydrogenation reaction conditions (413 K and 70 psig of H2 pressure) when using toluene or 1-octene as reactants, not affecting the activity under the steady state. Our computational studies also revealed that both crystal structures share common atomic cluster arrangements with small differences in angles and interatomic distances. The transformation was triggered by local distortions which favored shorter and more homogeneous interatomic distances as present in the hexagonal phase. The prepared molybdenum carbide materials were selective to the hydrogenation of 1-octene under the mild hydrogenation conditions tested. Hydrogenation of 1-octene was preferential in a 50 wt% mixture of toluene and 1-octene with minimal toluene hydrogenation under the tested conditions.